Presently, users seeking wireless telephony service typically subscribe to one of many providers of such service. Today's wireless telephony service providers not only offer voice calling but also offer General Packet Radio Service (GPRS) to enable the exchange data packets via a mobile wireless terminal. While GPRS exists in many areas, data transmission rates typically do not exceed 56 Kbs and the cost to wireless network service providers to support this service remain high, making GPRS expensive. To provide enhanced data communications, efforts now exist to establish new standards for wireless telephony. One such effort is the proposed “Universal Mobile Telecommunications System (UMTS)” standard specified by the 3rd Generation Partnership Project (3GPP) for advanced packet radio service in wireless telephony networks. The UMTS standard proposes transmission rates as high as 2 Mbps, making such service more attractive to subscribers.
In accordance with the UMTS standard, a subscriber's mobile wireless terminal will transmit its permanent identity, typically referred to as an International Mobile Station Identity or IMSI, to the network upon initial registration. To maintain user identity confidentiality and untraceability, after initial registration, each subscriber receives from the wireless network a temporary identity called Packet Temporary Mobile Subscriber Identity (P-TMSI) kept in a Serving GPRS Support Node (SGSN) in the network. Upon each subsequent access of the wireless network, the mobile wireless terminal will send its P-TMSI. The SGSN serving that mobile wireless terminal maps the P-TMSI to the user's permanent identity (i.e., IMSI). In this way, the user avoids transmitting its IMSI. To reduce the risk of breaching the user's confidentiality, the network should not identify the user for a long period by means of the same P-TMSI. Rather, the wireless telephony network should assign a new P-TMSI after some extended interval.
Despite efforts to protect the user's confidentiality via the P-TMSI, circumstances exist in present day wireless telephony networks that require the mobile wireless terminal to transmit its permanent identity (IMSI) after registration. For example, upon a new attachment to the wireless telephony network, the mobile wireless terminal could encounter an SGSN different than the one that served the terminal prior to detachment. Upon such a new attachment, the mobile wireless terminal will send its P-TMSI to the new SGSN. In turn, the new SGSN sends an Identification Request message to the old SGSN. If both the old and new SGSNs lack knowledge of the mobile wireless terminal, the new SGSN will send an Identity Request message to the terminal. The mobile wireless terminal must respond with is permanent Identity (IMSI) in clear text, breaching the confidentiality of the user's identity Thus, there is need for a technique for protecting the user's identity during attachment to a wireless telephony network.